This invention relates to a wire electrical discharge machining apparatus which cuts a workpiece by using a discharge phenomenon.
FIG. 6 is a perspective view showing a conventional wire discharge machining apparatus disclosed in Published Examined Utility Model Application No. 37544/1983. In FIG. 6, reference numeral 1 denotes a bed; 2, an X-Y cross table resting on the bed 1; 3, a workpiece; 4, a gate type column; 5 an upper arm; 6, a lower arm; 7, an upper guide; 8, a lower guide; 9, a storage drum for a wire electrode 12; 10, a tension applying device including a brake drum and a pinch roller; 11, a drawing-out and moving device having a capstan and pinch roller which draws out and moves the wire electrode 12; 13, an electrode head provided on a substantially the central top of the gate type column 4; and 14, a support for the workpiece 3 combined with the X-Y cross table 2.
The operation of the thus constructed conventional apparatus will be described. A gate type column section 4a is a single structure having the shape of a largely curved bow whose ends are provided with corresponding legs 4b connected integrally with the bed 1 and disposed on a base. The column portion 4a and legs 4b compose the gate type column 4. The electrode head 13 is fixed at substantially the center of the column portion 4a. The upper arm 5 is suspended from the head 13 or from the central top of the column portion 4a so that its vertical position is set freely. On the other hand, the lower arm 6 is provided so as to bridge the lower ends of the column portion 4a.
Under such arrangement, an electrical discharge removal process is performed between the workpiece 3 placed on the support plate 14 on the X-Y cross table 2 and the wire electrode 12 which is stretched and fed by the attractive force applying device 10 through a machining fluid therebetween.
It is said, according to the above structure, that the column portion 4a holding the upper and lower arms 5 and 6 is symmetrical in one axial direction with reference to the working shaft of the wire electrode 12, so that thermal equilibrium is maintained at least in one direction to reduce a deformation and/or deviation.
Since the conventional wire electrical discharge apparatus is constructed as just described above, the gate type column 4 necessarily large-sized, the need for the integral structure make it difficult to obtain the materials thereof and also makes machining difficult, which is a bottleneck in the manufacturing technique. The gate type column structure is also a bottleneck from the standpoint of a working space. Furthermore, the conventional apparatus only contemplates a deviation and movement of the wire electrode 12, but not a thermal deviation and movement of the workpiece 3, which is also important in order to ensure the machining accuracy. Namely, the support base 14 for the workpiece 3 is made of a stainless steel because of the need for corrosion resistance to the machining fluid. Although stainless steel has corrosion resistance, its coefficient of thermal expansion is about 17.times.10.sup.-6 /.degree.C., namely, large compared to that of a regular steel. Therefore, the workpiece 3 placed on the support base 14 is likely to deviate due to the thermal or temperature difference, so that the relative distance between the workpiece and the wire electrode varies to thereby result inevitably in a reduction of the machining accuracy.
The support base 14 for the workpiece 3 also has the problem that its surface will be highly damaged due to wear by the sliding of and shocks of the workpiece placed on the base. Generally, the workpiece 3 is often made of a tool steel or an ultra-high hard alloy. These high hard materials are especially likely to wear and damage the support base. Therefore, the workpiece on the support table 14 is difficult to rest horizontally, which brings about a noticeable reduction of the machining accuracy.